The present invention relates to integrated circuits, and, specifically, to impedance emulations that provide functions heretofore unavailable in integrated circuits.
The design and fabrication of integrated circuits has evolved to the point where virtually any digital or analog circuit may be created in silicon or other semiconductor material, and may be replicated in large numbers. Some aspects of circuit design have not been available to IC designers, due primarily to limitations imposed by physics and economics. For example, in some circuit designs there is a need for a relatively large capacitor, which may be fabricated in an IC only by forming parallel conductors in adjacent layers. If the required capacitor is large, the parallel conductors may occupy a significant portion of the total die area, thereby limiting the area available for the remaining components of the circuit. Since die size is linked to device cost, there is a strong economic incentive to avoid circuits that require a large capacitor.
Likewise, in IC circuit designs impedance is formed by various combinations of capacitance and resistance, due to the fact that there is no IC component that provides an inductive impedance. The multi-turn coil, well-known in classic electronics to create inductance, has no counterpart in the IC armamentarium. Although most circuits can be designed using capacitive reactance, the lack of integrated circuit inductance does comprise a design limit in IC layouts.
Assuming, arguendo, that it is generally not practical to incorporate large capacitors in an IC, and it is virtually impossible to incorporate inductors in an IC, there exists a need in the prior art for some way to replicate or emulate inductors and large capacitors, using the circuit components that are readily available in IC design.
The present invention generally comprises a method and device to replace circuit components that are difficult to form in integrated circuits, or that are too large in area to be economical, by providing component emulators that are simple to form with available IC components. The invention provides functionality, such as inductive impedance or large capacitance, that has been heretofore unattainable in integrated circuits. Furthermore, the invention may provide significant reductions in the die size of integrated circuits. Values of components that are difficult or impossible to integrate can thus be put in silicon.
The method of the invention employs a pair of impedances connected in series across a potential difference, the impedances forming a voltage divider having at its midpoint a reference voltage VX. An equivalent impedance pair network is connected across the potential difference, and the two networks are connected in an H configuration, with the cross-link joining nodes that are both at VXxe2x80x94. The impedances are scaled, so that the first pair have values of KZa and KZb, and the second pair of impedances have values of KZa/(Kxe2x88x921) and KZb/(Kxe2x88x921).
Any one of the impedance may be replaced by an operational amplifier (OP AMP) having unity gain, in which the output of the OP AMP is connected to the remaining impedance of the modified leg, and the positive input of the OP AMP is connected to the VXxe2x80x94node of the unmodified leg. The negative input of the OP AMP is connected to the output thereof in a direct feedback loop. The unity gain OP AMP forces the output voltage thereof to follow the input voltage VX, as if the impedance had not been removed. By proper choice of components and values, the impedance that is replaced may comprise a large capacitor, and the remaining impedances may comprise resistance and small capacitance, both of which, together with the OP AMP, are easily integrated in a small die area. Thus the output VX of the OP AMP may be made to emulate the behavior of a large capacitor, in effect forming a virtual capacitor. And the components used in the circuit occupy a die area far smaller than the capacitor that has been replaced.
In another aspect, the impedance emulator described above may be modified by the addition of a transistor having a source/drain circuit connected in place of the eliminated impedance of the H network, with the OP AMP output connected to the transistor gate. The negative input of the OP AMP is connected to the junction of the transistor source/drain circuit and the remaining impedance. This arrangement relieves the OP AMP from acting as a source or drain for the remaining impedance, thereby eliminating unacceptably large current flow through the OP AMP.
In a further aspect, the invention may be configured to emulate an inductive impedance. Using the general circuit arrangement described above, the OP AMP may be provided with a negative gain, and the impedances may be scaled to create a virtual inductor having a predetermined value.
Additionally, the invention may include at least one variable impedance, such as a variable resistor, whereby the emulated impedance may comprise a variable capacitance or a variable inductance.
Impedance emulator circuits disclosed herein may be combined so that the emulated impedance of one circuit may act as a virtual component in a further emulation circuit, whereby a wide range of component impedances values of may be replicated. For example, an inductor emulator circuit may employ a large capacitance to achieve a large inductance value. The large capacitance, which is impractical in an integrated circuit, may be comprised by a capacitor emulator circuit connected appropriately in the inductor emulator circuit. Other such combinations of emulator circuits may be made to create a wide range of capacitor and inductor values.
We have previously disclosed a setup in which one impedance emulator acts as an impedance for another emulator, in a chained effect. A chained emulator works equally well in a floating connection as in a grounded circuit Sometimes large capacitors are required to handle large currents, at least instantaneously. There is no difference in the current handling ability of an emulated impedance versus an actual impedance. In the emulated impedance the current handling task is being assigned to the amplifier. For example, assuming a very large capacitor 1 F (one farad) has a finite lead resistance of 0.1 ohmand a parasitic RC time constant of 0.1 seconds. We may use this parasitic resistance: a 100 ohm resistor in series with a smaller capacitor (1 mF) is used, creating the same RC time constant RC=0.1 sec., yet with much less current handling capability. Note that the amplifier will have to drive the 0.1 ohm resistor, nearly a short-circuit for the amplifier. The resistor ratio of 100/0.1=1000 performs, via the amplifier, a capacitor multiplication factor of approximately 1000.